I'll just dump all of my thoughts about it here for the moment.
There would be 2 teams. Maybe 5 players per team.
A goal on each end of the map would be what each team aimed for of course.
Every player in the game would have a grav. gun. I'm not sure, however, how say Dog's ball, or any object for that matter, would react to being pulled on by multiple grav. guns at the same time. There could be an issue with that.
First team to a certain number of points would win, or it could also be timed I imagine.
Ideally this would probably be a multiplayer game. The AI for such a game to be played well would probably be pretty hard.
It would be cool, and take a bit of getting used to, to pass the ball to a teammate, but a member of the opposing team could intercept the pass from a great distance away, or just alter the direction of the ball with the grav. gun. This would add a new difficulty to just getting it into the basket.
This seems like something that would be cool to take on, i'm just not sure what effect 10 gravity guns would have on a ball.